FR2574946A1 - METHOD FOR RADIOELECTRIC SYNCHRONIZATION OF SLAVE STATIONS BY A MASTER STATION, IN PARTICULAR FOR A MLS-TYPE LANDING SYSTEM, AND DEVICES FOR IMPLEMENTING SUCH A METHOD - Google Patents

Abstract

THE INVENTION CONCERNS A PROCESS FOR RADIO-ELECTRIC SYNCHRONIZATION OF SLAVE STATIONS BY A MASTER STATION, IN PARTICULAR FOR A LANDING ASSISTANCE SYSTEM OF MLS TYPE. THE PROCESS CONSISTS OF SENDING BY THE AZIMUT STATION, BEFORE EACH MLS CYCLE CONSTITUTED BY A PLURALITY OF SUCCESSIVE EMISSIONS OF MLS FUNCTIONS, A SYNCHRONIZATION INFORMATION CODED IN THE FORM OF A SERIES OF SHORT-TERM PULSES. APPLICATION TO RADIO-NAVIGATION.

Description

METHOD FOR RADIOELECTRIC SYNCHRONIZATION OF SLAVE STATIONS

  BY A MASTER STATION, IN PARTICULAR FOR A SYSTEM

  MLS TYPE LANDING AID AND DEVICES

IMPLEMENTING SUCH A METHOD.

  The present invention relates to a method for the radio-telehronronization of slave stations by a master station, which method is particularly applicable to the synchronization of the azimuth stations and the site of an MLS type landing aid system. The invention also relates to devices for implementing said method.

  It is recalled that an MLS landing aid system -

  Microwave Landing System, according to the Anglo-Saxon expression -

  i0 provides an aircraft with different information - called "functions" - on its position, including the azimuth angle and the angle of

  site in a landmark linked to the airstrip and, where

  other related functions such as the rear azimuth for example, and a certain number of data, some called "basic" and the others called "auxiliary". These different informations are emitted alternately by the MLS system, from the ground in time multiplexing on the same frequency, close to 5 GHz,

  according to characteristics standardized by the Avia-

  International Civil Aviation Organization (ICAO), Annex 10, paragraph 3-Eleven.

  Each of these information items, on the azimuth or the site in particular, is divided into two parts, successively issued: - a preamble whose main role is to provide the aircraft with an identification of the program that will follow immediately. This preamble is emitted by a so-called sectoral antenna, that is to say a fixed radiating pattern antenna covering the entire area, or sector, covered by the MLS system. The preamble is in the form of a binary word transmitted in Differential Phase Shift Keying (DPSK) according to ICAO standards; the actual angular information, emitted by means of an electronic scanning antenna, according to the well-known principle of the Time Reference Scanning beam.

Beam, TRSB, in English).

  Figure I shows a frequent arrangement of the stations

  azimuth, site and rear azimuth around an airstrip.

  The azimuth and site stations are generally placed relative to one another at a distance of several kilometers. The azimuth station (Az) is close to the end of the track, marked P and ZZ axis, and it emits towards the P track thus allowing the aircraft (Av) to have the angular information of during the entire landing, even during the rolling phase on the runway. The site station (S) emits in the same direction but it is close to the threshold of the runway, allowing the aircraft (Av) to be guided on a constant angle of elevation trajectory during the landing. to be brought by this trajectory at the beginning of the track. The rear azimuth station (AAR) is placed at the runway entrance, symmetrically at the azimuth station Az, and also emits towards the runway. Timeshare operation on a

  same frequency therefore imposes the existence of a link

  chronization in time between stations, to ensure non-compliance

  azimuth, site and rear azimuth overlap.

  FIG. 2 represents an exemplary embodiment of a se-

  complete ICAO time period issued by an MLS system.

  It consists of a succession of identical cycles of duration equal to 615 ms maximum. Each cycle itself is composed of

  a succession of "sequences" and auxiliary data words.

  Figure 3, a and b, respectively represent two particular examples of constitution of what is called "sequence", of type 1 and 2. In these two types, it comprises a sequence of time slots respectively reserved for the azimuth, at the site, at the aft azimuth

and basic data.

  The role of synchronization is to ensure that the various functions, azimuth, site, rear azimuth, are transmitted in the time slot allocated to them. In general, the azimuth station plays the role of master station by generating the MLS cycle. For this, it must send synchronization signals to others

  stations to allow them to transmit at the desired time.

  This synchronization can be achieved by physical connection, electric cable, optical fiber for example. But it has the disadvantage of being expensive because of the civil engineering works it involves, including the construction of trenches over distances of up to several kilometers. That's why he

  seems more interesting to achieve this radio synchronization

1 5 trimming.

  A solution to achieve this radio synchronization

  using the round-trip scanning of the azimuth station beam, the principle of which is already mentioned is described in more detail below, in connection with Figures 3 and 4 which illustrate the

  emission principle respectively azimuth and site.

  From an azimuth station, according to the foregoing, two different radiations are emitted by two separate antennas, that for

  simplify is shown in one point Az in Figure 4.

  Starting from the point Az, there is therefore on the one hand the emission diagram of the preamble, noted PAZ 'emitted by a sectoral antenna throughout the coverage area of the MLS system, which is represented in the figure by an angle ac. From this point Az we have, on the other hand, the diagram of a flat and vertical BAZ beam, called the flying beam, emitted by an electronic scanning antenna; the beam BAZ carries out at a constant speed a forward scan then, after a stop time, a return scan, and this in a scanning zone making an angle B z in FIG. 4, which may be equal to or less than the angle previous cover. In the figure, z is shown to be smaller than; Az and arrow Rz have also been shown respectively with the forward scan and scan paths around the BAZ beam in the scan area. Finally, we have figured a plane Av not correctly aligned, for example, with the axis ZZ of the track. According to ICAO standards, the angle $ Z is between 20 minimum, decomposing with respect to the ZZ runway axis into two half-angles -0M = + OM = 10, and 120 maximum with -0M = + 0M = 60; the BAZ beam has an opening of 1 to 4 in the plane of the

  figure and about 15 in the vertical plane.

  FIG. 5 shows, in a similar way to what has been

  been done in Figure 4, the principle of the website emission.

  FIG. 5 thus distinguishes the site station S from which two beams are emitted by two distinct antennas: one sectorial, one emitting the site preamble P5, of which the diagram has been represented in the figure, and the other emitting a flat bending beam B5, scanning swept area 8S, sweeping performed

  as for the azimuth beating beam BAZ.

  In this first type of solution, the round-trip scanning of the beating beam of the azimuth station, which passes necessarily on the site station, is used for the synchronization of this last station. This implies that the site station is further equipped with: - a reception antenna, operating at approximately 5 GHz according to ICAO standards, directed towards the azimuth station so as to capture the beating beam of this station during its passage;

  - receiving means, operating at approximately 5 GHz, detect

  both the pulses sent by the beating beam of the azimuth station;

  logical means for retrieving the sequences of the

  3 0 sions; - a clock with high accuracy and stability to ensure the autonomy of the operation of the station

  site, even in the presence of a momentary interruption of the

  of the signal emitted by the azimuth station - logical means for carrying out the transmission of the sequences

  specific to the site station in the event of a momentary loss of information

synchronization.

  A first type of inconvenience of this solution is due to the rounded shape and the large width of the pulses obtained by the round-robin passage BAZ on the site station (50 to ls), the slow edges of which do not allow to obtain a good accuracy on the arrival time of the pulse, precision that

  we want the order of a few lilies.

  A second type of inconvenience comes from the great

  the complexity of the logical means for retrieving the transmission sequences, given that the azimuth and site sequences follow one another in a non-regular manner in an MLS cycle that forces the station

  slave (site) to distinguish them from each other.

  A second solution to the problem of radio synchronization

  The station's electrical station by azimuth station consists of using the preamble issued by the station. In this case, the site station must have the same additional means as in the first type of solution, except that the receiving means

  must now decode modulated transmissions in DPSK phase.

  Here again, such a solution has drawbacks, including the need for a very sensitive MLS receiver, because the emission of the preamble by the sectoral antenna is at a lower level.

  transmission by the scanning antenna, and the complexity of the reception

  which must be able to decode the DPSK phase modulation.

  The purpose of the present invention is to overcome these various

  disadvantages. The subject of the invention is a synchronization method

  radio stationation of at least one slave station by a station

  master in a system of communication of information,

  a plurality of successive information transmission steps by the different stations constituting a cycle, the method comprising an additional step of transmission by the master station of coded synchronization information in the form of a plurality of successive pulses, issued periodically, at least once

per cycle.

  The subject of the invention is also means for implementing

  of this process, on transmission and reception.

  Other objects, features and results of the invention

  will appear in the description that follows, given as a non-exhaustive example.

  limiting and illustrated by the following figures which, in addition to Figures 1 to 5 already described, represent: - Figure 6 a and b: an example of a synchronization signal used in the method according to the invention; FIG. 7: an embodiment of the device for implementing the invention on transmission in the case of an MLS system; FIGS. 8 and 9 show an embodiment of the device for implementing the invention on reception in the case of an MLS system;

  - Figure 10: an improvement of the invention.

  Elements having the same function with a view to the same result

  have the same reference in the different figures.

  According to the method that is the subject of the invention, in an MLS system, the synchronization of at least one slave station by a master station is performed by an additional step of transmitting synchronization information encoded in the form of pulses. of

  the short term is clearly different from all other information

  emitted by these stations in normal operation.

  The role of master can be played by any one of

  MLS system stations (azimuth, site, rear azimuth, data ...).

  However, for coverage reasons, this role is preferably assigned to the azimuth station. The other stations of the

  MLS systems are then slave stations.

  On the other hand, two embodiments are described below.

  below, as to the timing of the transmission of this synchronization information with respect to the normal transmission of the MLS functions by the different stations. In the first mode, the synchronization information is transmitted at the beginning of each MLS cycle and in the second mode, it is at the beginning of each function, for each

slave station to synchronize.

  Finally, the synchronization information is issued either by

  the sectoral antenna, ie by the electronic scanning antenna.

  0 When the emission is made by the sectoral antenna, the radiation pattern covers both the site station and the azimuth station

  back. However, the radiation of these synchronization pulses

  will be preferentially transmitted by the scanning antenna.

  tronic, which has the advantage of a great gain and a good directivity. When the synchronization information is for example transmitted at the beginning of each MLS cycle, but on the same frequency, close to 5 GHz. In this case, to achieve the synchronization of the site and rear azimuth stations, for example, from the At the azimuth station, the scanning antenna must be pointed at each of them successively every two MLS cycles, at the beginning of each of them. Such a periodicity is sufficient in the measure o

  each of these slave stations comprises memory means

  the flow of references within an MLS cycle and means for saving or autonomy of this information

  for a time equivalent to several hundred MLS cycles.

  More generally, in the case of the synchronization of N slave stations by a master station, the scanning antenna is pointed

  to each of them all N MLS cycle starts.

  According to the second embodiment o the synchronization is done at the beginning of each function for each of the slave stations to be synchronized, the synchronization information is transmitted in the same way as before either by the sectoral antenna or by the antenna. electronic scanning, which is then pointed to each of the stations to be synchronized at the beginning of each function and transmits synchronization information. However, the directivity

  beam is usually not sufficient to address

  each of the slave stations, it is preferable that the information be

  In this case, the synchronization code is coded differently according to the slave stations. The number of codes required is equal to the number

  N of slave stations to be synchronized by the master station.

  Thus, according to the method which is the subject of the invention, the azimuth station 10 of an MLS system transmits, from one of the antennas which it necessarily has to fulfill its function, synchronization information coded in the form of minus two pulses, the

  coding to be different from that of the trans-

  put by the MLS system in its normal operation.

  Two examples of timing signal are shown in Figures 6a and b. The synchronization signal uses pulse triplets at MLS frequency, about 5 GHz, amplitude modulated by all or nothing. The pulses, identical, have a width ú = 20 lis so as to clearly distinguish them from the pulses of the beating beam (50 to 200 ls). The coding is given by the spacing of these three pulses. Two examples of coding are given respectively in FIGS. 6a and 6b. The synchronization time reference is for example the rising edge of the

first impulse.

  The synchronization signals can be used in various ways: only the signal of FIG. 6a emitted at the beginning of the MLS cycle of 615 ms is used. The slave station (site or rear azimuth) must then contain a sequencer that rolls the entire MLS cycle, so as to know the time periods allocated to it - the signal of Figure 6a is used for the azimuth-site synchronization and the signal of Figure 6b to the rear azimuth-azimuth synchronization. The appropriate timing signal (FIGS. 6a or 6b) is emitted at the beginning of each function to be synchronized (rear aziumut site) and causes the transmission of a single function. This system takes longer than the previous one for synchronization; it also requires a sequencer that rolls out the MLS cycle, and a logic system that allows for momentary loss of the synchronization signal. any intermediate solution between the two preceding ones can be used, for example by emitting the signal of FIG. 6a at the beginning of the sequence 1 and the signal of FIG. 6b at the beginning of the

sequence 2 (see Figures 2 and 3).

  Moreover, the shape of the pulses must be chosen to have on the one hand a short rise time (qq us) to ensure a

  good synchronization accuracy and secondly a

  die to ensure a rapidly decreasing spectrum around the

carrier frequency.

  The synchronization pulses emitted by the antenna of the

  station azimuth, which are short-term and without preamble -

  unlike other information issued by the station - are not decodable by MLS receivers, located on board aircraft, which are not disturbed by this information (useless for them). Moreover, these impulses come out of period

  MLS information, so that they do not interfere.

  Finally, insofar as each series of short pulses is transmitted at most only once by transmission of a slave function,

  they have no disturbing effect on the spectrum.

  The azimuth station on the one hand and the site and rear azimuth stations for example on the other hand, must be provided with special means to transmit and receive. These means are

described below.

  FIG. 7 represents an embodiment of the device for implementing the invention on transmission, in the case of an MLS station. This device essentially comprises a transmitter 1, two antennas: a sectoral antenna 3 and a scanning antenna

  electronics 4, and different control circuits (2,5).

  Transmitter I comprises, in cascade: a frequency generator 11, constituted for example by a frequency synthesizer providing a wave close to 5 GHz according to the ICAO standard (it is recalled that, according to this standard, a frequency among 200 predefined frequencies adjacent to 5 GHz is assigned to each MLS station); a phase modulator 12, performing a two-state DPSK phase modulation (O, r), for transmitting the preamble and the data on command of a control logic device 5, 1 0 such as a microprocessor; a device 13 for on / off control, also controlled by the microprocessor 5; a power transmitter 14, produced with a tube or with transistors according to the required power, which is conventionally of the order of 20 W.

  and therefore most often realized using transistors.

  The transmitter 1 provides a signal, via a

  mutator 2, either on a V1 channel to sectoral antenna 3 for the transmission of the preamble and data, basic or auxiliary, or

  on a V2 channel to the scanning antenna 4.

  The electronic scanning antenna 4 is broken down into a power divider (or splitter) 41, dividing the power received from the switch 2 into N in order to supply N digital phase shifters (block 42), which supply N radiating elements (block 43 ); the

  The values of the phase shifts introduced by the phase shifters 42 are

  driven by a scanning logic circuit 44, in order to perform electronic scanning from static radiating elements, as is known. It will be recalled that, if it is desired to emit a beam (of wavelength λ) at an angle θ with the normal to the alignment of the radiating elements 43, the phase shift λ i introduced by a phase shifter of rank λL (λ <. N) is given by: d A (i 2 ir i .. sin 0 od is the distance between two successive radiating elements The scanning logic 44 is generally a wired circuit, distinct from the microprocessor 5 (but this is not not imperative) due to the speed required, this circuit 44, on the order of departure of the microprocessor 5, controls the phase-shifters 42 so as to ensure the desired scanning by rapid successive pointing of the antenna lobe; phase shifters 42 for each clocking are generally stored (memory 45) in memories of the PROM type The number N of the phase shifters 42 is

commonly in the range of 20 to 100.

  The entire device is controlled by the micropro-

  5 and its memory 50 (of the PROM type for example), connected to the transmitter 1, to the switch 2 and to the scanning antenna 4, by

  via the scanning logic circuit 44; the micropro-

  In the case of the azimuth station, on the other hand, in the case of the azimuth station, the receiver 5 supplies the scanning logic circuit with the starting time of the scan for the site or azimuth functions (arrow 51). from the beating beam to the site station during the transmission of the synchronization signal (arrow 52). The MLS cycle, as represented by

  FIGS. 2 and 3 are contained in the memory 50 of the micropro-

ceaser 5.

  More precisely, according to one embodiment, a "status word" of the station is defined in the microprocessor 5, each bit of this word representing a command, in the example above, this word comprises at least six bits. , respectively controlling: - the modulator 12; - the command 13; - the switch 2; scan logic 44 (2 bits);

  - Site synchronization (Ss) in the case of the azimuth station.

  The real-time generation of a sequence, such as that of FIGS. 4b or 5b, amounts to generating in real time this status word, which then realizes the various commands. For this purpose, the sequence of the status words corresponding to the desired sequence is stored in a table of the memory 50. At the rate of a suitable clock (here 64 ls), the microprocessor 5 fetches the successive status words and supplies them to an interface (not shown), of the PIA (for "Parallel Interface Adapter") type, for example, which controls the various blocks. It should be noted that, due to the repetition of some of the functions in the same sequence, it is

  possible to reduce the necessary memory space in the hierarchy.

  chisant: we then memorize as many tables as functions, a main table defining the sequence by calling successive functions. An MLS system comprises, on the ground, in general three stations such as that of FIG. 7, as illustrated in FIG. 1: one for the azimuth function, one for the site function and one for the

rear azimuth function.

  According to the invention, that of the stations which is chosen as master station (azimuth station for example) then sees the contents of its memory 50 modified to ensure the additional transmission of

synchronization pulses.

  FIG. 8 represents an embodiment of the additional means necessary for the implementation of the invention in the

  reception in a slave station.

  This device comprises, in cascade: - an antenna 60, capable of receiving the radiation of the master station at about 5 GHz - a band-pass filter 61, centered on the previous frequency

  and followed by an amplifier 62 having the same operating frequency

  ment; a device 63 for detecting the signal of synchronization and shaping of this signal, with diodes for example;

  a device 64 for decoding the synchronization information

  sation, which sends a signal giving the synchronization reference time to the control device 5 of the slave station in question. In an alternative embodiment, the filter assembly 61 and

  amplifier 62 can be replaced by a conventional MLS receiver

  tional (superheterodyne 200 channels).

  FIG. 9 represents an embodiment of the decoder 64 of FIG. 8, in the case where the synchronization information is in the form of three pulses such as those of FIG.

Figure 6a.

  This decoder comprises a shift register 70, controlled by a clock 72 of frequency equal to 100 kHz, having 15 positions which allows it to decode a signal received from the detector

63, 0.15 ms.

  Each of the positions of the register 70 is connected to an AND logic gate 1o 71, possibly via an inverter 73,

  programming the message to be decoded by the position-

  The AND gate 71 then carries out the correlation between the expected message and the received message: its output is only at the reception of the correct message. This output provides the synchronization reference instant to the control device 5. FIG. 10 shows an improvement in the operation of the slave stations in the event of an accidental and momentary interruption of the radio-electric link between the master and slave stations. Such an interruption may be due simply to the presence of a moving obstacle (passage of a

jumbo jet for example).

  The device of FIG. 10 is a clock system (clock 80 and counter 81) slaved in frequency on the synchronization signal received from the decoder 64, and designed so that it continues to deliver a signal to the control circuit 5 even in the absence of signal

received from the decoder 64.

  For this purpose, this device comprises - a clock 80, with good stability over time and with

  high frequency to have a good accuracy in the enslaving

  ment; a time-controlled quartz oscillator giving an accuracy of the order of 10-6 may be suitable; a counter 81 modulo N, so as to generate a signal of period T = 615 ms which is the duration of an MLS cycle. If we use a clock of period T = 0.1 its, we obtain a number N of 7 o counting such that No = 0, 615 x 10 7 In order to take into account a possible drift of the clock 80 over time , due for example to temperature variations, a number N different from No is chosen, the change of a unit of the number N then corresponding to a frequency variation of +

1, 6 3. 10-7.

  For this purpose, an RS-type flip-flop 86 serves as a phase discriminator and whose output state is representative of the phase advance of one of the signals it receives with respect to the other, that is to say the input signal (from the device 64) and the output signal (from the counter 81); a down-counter 83, modulo 2PK; the output 1 5 has p bits of low weight, which are not taken into account, and

  bits of high weight which represent a number K. This counter-

  up-converter 83 sees its contents vary from + 1 to each period TO according to the relative phase of S1 and S2. However, it is necessary to successively have 2P times the value + 1 to vary K of + 1I: this corresponds to a low pass filtering effect in the loop; an adder 82 performing the sum N = N'o + K, with N'o fixed value corresponding to the lowest value of N to be considered, corresponding to the extreme clock frequency to compensate for

  K ser, that is, No = N'o + 2, where Ko is the maximum content

of the down-counter 83.

  The circuit as described above thus provides a

  phase (and therefore frequency) of the output (to circuit 5) on the input (from the circuit 64). A monostable 85, connected between the input and up-down counter 83, sends a count-down count to element 83 for a period of time equal to its period. This period (TM) being chosen such that To <TM <2 TB, it appears that if a signal is received at the input permanently,

  enslavement works but that if nothing is received, the enslaving

  is interrupted after the first missing pulse. The circuit then stores the last value of N and continues to

  providing a signal, of period To, to the device 5.

  Finally, the circuit of FIG. 10 comprises an initialization command of the counter 81 at the value N = No, and a delay element 84, connected between the input of the circuit and the input of the up-down counter 83, intended to compensate for the delays experienced by

  the input signal in the flip-flop 86.

Claims (14)

  1. Method of radio synchronization of at least one   slave station by a master station in a communication system.   cation of information comprising a plurality of successive information transmission steps by the different stations constituting a cycle, the method being characterized by the fact that it comprises a   an additional step by the master station   encoded synchronization in the form of a plurality of pulses   successive sessions, issued periodically at least once per cycle.   i 0 2. Method according to claim 1, characterized in that the communication system is an MLS system comprising a   plurality of successive steps for transmitting MLS functions   killing a cycle, and that synchronization pulses are emitted   the transmission frequency of the MLS functions and have characteristics   different from the other information emitted by the stations. 3. Method according to claim 2, characterized in that the synchronization pulses are emitted at the beginning of each MLS cycle.   4. Method according to claim 2, characterized in that the pulses forming the synchronization information are transmitted, for each MLS cycle, at the beginning of each function, at their destination.   of the slave station transmitting this function.   5. Method according to one of claims 2 to 4, characterized by   the fact that the master station is the azimuth station of the MLS system.   6. Synchronization method according to one of the claims   preceding, by a master station with a sector antenna   and an electronic scanning antenna, characterized in that the synchronization pulses are emitted by the sectoral antenna.   7. Synchronization method according to one of claims I   to 6, N slave stations by a master station, having a   sectoral antenna and an electronic scanning antenna,   in that the synchronization pulses are emitted by the electronically scanned antenna pointed successively to each of the slave stations.   8. Method according to one of the preceding claims, characterized   realized by the fact that the coding of the synchronization information   is achieved by the spacing between the pulses.   9. A method according to any one of the preceding claims   are characterized by the fact that the synchronization information is   encoded as a series of three pulses.   10. Device for implementing the procedure according to   one of the preceding claims in a master master station   concerning: - a sectoral antenna tO; - An electronic scanning antenna (4) - transmitter means (1), providing the information to be transmitted to the previous antennas; switching means (2) between the emitting means (1) on the one hand and the two antennas (3,4) on the other hand; control means (5), ensuring the sequencing of the emissions from the station.   The device being characterized by the fact that the means of   command (5) furthermore provide control of the additional step synchronization.   11. Device for implementation at the reception of the process   according to one of claims I to 9, in a slave station   having control means (5) for sequencing station transmissions; the device being characterized in that it further comprises: an antenna (60) for receiving the synchronization information; means for receiving and detecting synchronization information (61, 62, 63); decoding means (64) of the signal coming from the preceding means, supplying, if necessary, a signal giving the synchronization reference instant, called the synchronization signal, to the   control means (5> of the station.   12. Device according to claim I1, characterized by the fact that the receiving and detecting means comprise a filter (61) and a circuit for detecting and shaping (63) the filtered information.   13. Device according to one of claims 11 or 12, characterized   in that the decoding means (64) includes a shift register (70) connected to an AND gate via   inverters (73) arranged at certain outputs of the different posi-   the register according to the configuration of the sync pulses. to decode.   14. Device according to one of claims 11 to 13, characterized   in that it furthermore comprises means for generating the synchronization signal in the absence of reception of the synchronization information transmitted by the master station, these means being placed between the decoding means (64) and the control means (5) of the station, and comprising a system   frequency-controlled clock (80, 81) on the synchronization information   where it exists, delivering the synchronization signal to the   control means (S) of the station.